Golf ball core with tailored hardness gradient

ABSTRACT

Compositions including a hardness agent and golf ball cores made from such compositions having a tailored hardness gradient are disclosed. The type and concentration of the components in the composition, including the hardness agent, affects the hardness, hardness gradient, and compression of cores made from the composition and, thus, can be used to produce a golf ball having desirable performance characteristics.

FIELD OF THE INVENTION

The present disclosure relates generally to compositions for use in golfball cores that provide for manipulation of the hardness gradient ofsuch cores. More particularly, the present disclosure providescompositions and golf ball cores made from such compositions thatprovide an ability to tailor and/or improve certain ball performanceresults when such cores are used in a golf ball. In some respects, thepresent disclosure relates to golf ball cores with tailorable hardnessgradients that, when used in golf balls, provide the ability to achieveone or more desired performance characteristics including, for example,spin on driver and short distance shots.

BACKGROUND OF THE INVENTION

The performance of a golf ball is affected by a variety of factorsincluding the materials, weight, size, dimple pattern, and externalshape of the golf ball. As a result, golf ball manufacturers areconstantly improving or tweaking the performance of golf balls byadjusting the materials and construction of the ball as well as thedimple pattern and dimple shape.

For example, the resiliency and rebounding (Coefficient of Restitution)performance of the golf ball are generally driven by the composition andconstruction of the core. The spin rate and feel of the ball also areimportant properties that are affected by the composition andconstruction of the core. The spin rate refers to the rate of rotationof the golf ball after being hit with a club. Two factors that affectthe spin rate of a golf ball are the hardness gradient of the core,i.e., the difference in hardness between the geometric center and outersurface of the core, and the compression of the core, i.e., how much thecore deflects under a given load. Generally, changing the compression ofa golf ball or golf ball core alters the spin rate of the golf ballsimilarly for both driver shots and short distance shots. For example,increasing the compression of a golf ball or golf ball core may cause a10 percent increase in spin rates for both short distance and drivershots. In contrast, changing the hardness gradient of the golf ball coremay affect the spin rate differently for driver shots and short distanceshots. More specifically, an increase in the hardness gradient mayresult in a 15 percent increase in spin rate on short distance shots butonly a 5 percent increase in spin rate on driver shots.

Most professionals and highly skilled amateurs (i.e., those who cancontrol the spin of a golf ball) generally prefer balls with high spinrates to allow for better control in and around the green and draw andfade on approach shots. Indeed, these balls are beneficial for shortdistance shots made with irons and wedges. In contrast, recreationalplayers who cannot necessarily control the spin of the ball will likelyfind that it is easier to play with a golf ball with low spin becausethe spin from a golf ball with a high spin rate can create more shotdispersion, i.e., more stray off to the left or right of the centerline,especially if the ball is hooked or sliced.

Meanwhile, the “feel” of the ball generally refers to the sensation thata player experiences when striking the ball with the club. Most playersprefer balls having a soft feel, because the players experience a morenatural and comfortable sensation when the club face makes contact withthese balls. Balls having a softer feel are particularly desirable whenmaking short shots around the green, because the player senses more withsuch balls. The feel of the ball primarily depends upon the hardness andcompression of the ball.

Accordingly, there remains a need for golf ball cores having a hardnessgradient that can be tailored to produce desired performancecharacteristics such as resiliency, rebounding, spin, and feel. In thisaspect, it would be advantageous to tailor the core of a golf ball suchthat the finished golf ball has the desired amount of spin on shortdistance and driver shots for players having different levels ofexpertise as well as other advantageous properties, features, andbenefits. For example, it would be beneficial to tailor the core of agolf ball so as to reduce shot dispersion for amateur players.Similarly, it would be advantageous to tailor the core of a golf ballsuch that the finished golf ball gives highly skilled players greatercontrol over the shot. The present disclosure provides compositions foruse in golf ball cores and golf balls containing such cores that allowfor manipulation of the hardness gradient and, thus, manipulation ortailoring of desired performance characteristics.

SUMMARY OF THE INVENTION

The problems expounded above, as well as others, are addressed by thefollowing inventions, although it is to be understood that not everyembodiment of the inventions described herein will address each of theproblems described above.

In some embodiments, the present disclosure provides a golf ballincluding a core and a cover layer disposed about the core, the coreincluding: a rubber formulation including a base rubber and a hardeningagent having a hardening agent concentration HA_(C) and a hardeningagent isomer number HA_(LN); a geometric center hardness H_(C); asurface hardness H_(S); and a hardness gradient H_(Gr) equal to thedifference between H_(C) and H_(S), wherein the hardening agent isnitrophenol, and wherein

$\frac{{HA}_{C}*{HA}_{LN}}{1 - \frac{1}{H_{Gr}}} \geq {0.4.}$

In another embodiment,

${0.6} < \frac{{HA}_{C}*{HA}_{LN}}{1 - \frac{1}{H_{Gr}}} < {{0.6}{3.}}$

In yet another embodiment,

${1.5} < \frac{{HA}_{C}*{HA}_{LN}}{1 - \frac{1}{H_{Gr}}} < {1.8.}$

In still another embodiment,

${2.0} < \frac{{HA}_{C}*{HA}_{LN}}{1 - \frac{1}{H_{Gr}}} < {2.5.}$

In some embodiments, HA_(C) is about 0.1 to about 1.0 parts per hundredrubber. In one embodiment, H_(C) is in the range of about 50 Shore C toabout 85 Shore C, H_(S) is in the range of about 65 Shore C to about 95Shore C, and H_(S) is greater than H_(C). In another embodiment, therubber formulation further comprises a co-agent, a filler, and aninitiator. In yet another embodiment, the rubber formulation furthercomprises a radical scavenger. In still another embodiment, the baserubber is polybutadiene rubber, butyl rubber, or a blend thereof.

In other embodiments, the present disclosure provides a golf ballincluding a core and a cover layer disposed about the core, the coreincluding a rubber formulation including a base rubber, an initiatorhaving an initiator concentration I_(C), and a hardening agent having ahardening agent concentration HA_(C) and a hardening agent isomer numberHA_(LN); a geometric center hardness H_(C); a surface hardness H_(S);and a hardness gradient H_(Gr) equal to the difference between H_(C) andH_(S), wherein the hardening agent is nitrophenol, and wherein

$\frac{HA_{C}*HA_{LN}}{1 - \frac{I_{C}}{H_{Gr}}} \geq {0.6.}$

In another embodiment,

${{0.6}2} < \frac{HA_{C}*HA_{LN}}{1 - \frac{I_{C}}{H_{Gr}}} < {{0.6}{4.}}$

In yet another embodiment,

${1.6} < \frac{HA_{C}*HA_{LN}}{1 - \frac{I_{C}}{H_{Gr}}} < {2.0.}$

In still another embodiment,

${2.4} < \frac{HA_{C}*HA_{LN}}{1 - \frac{I_{C}}{H_{Gr}}} < {2.7.}$

In one embodiment, HA_(C) is about 0.1 to about 1.0 parts per hundredrubber. In another embodiment, I_(C) is about 0.5 to about 3.0 parts perhundred rubber. In still another embodiment, H_(C) is in the range ofabout 50 Shore C to about 85 Shore C, H_(S) is in the range of about 65Shore C to about 95 Shore C, and H_(S) is greater than H_(C).

In still other embodiments, the present disclosure provides a golf ballhaving a core and a cover layer disposed about the core, the coreincluding: a rubber formulation including a base rubber and a hardeningagent, wherein the hardening agent is a benzoic compound comprising afirst functional group that is a nitro functional group and a secondfunctional group that is selected from the group consisting of hydroxyl,amino, and sulfhydryl functional groups; a geometric center having ahardness; a geometric surface having a hardness; a hardness gradientequal to the difference in the geometric center hardness and the surfacehardness, wherein the hardness gradient is between 2 Shore C and 42Shore C.

In one embodiment, the hardening agent is nitrophenol. In anotherembodiment, the hardening agent is 2-nitrophenol and the hardnessgradient is between 30 Shore C and 42 Shore C. In yet anotherembodiment, the cover layer comprises a material selected from the groupconsisting of polyurethanes, polyureas, and hybrids, copolymers, andblends thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features and advantages of the invention can be ascertained fromthe following detailed description that is provided in connection withthe drawings described below:

FIG. 1 is a cross-sectional view of a two-piece golf ball in accordancewith an embodiment of the present disclosure;

FIG. 2 is a cross-sectional view of a three-piece golf ball inaccordance with an embodiment of the present disclosure;

FIG. 3 is a cross-sectional view of a four-piece golf ball in accordancewith an embodiment of the present disclosure; and

FIG. 4 is a cross-sectional view of a five-piece golf ball in accordancewith an embodiment of the present disclosure.

DETAILED DESCRIPTION OF THE INVENTION

The present disclosure relates to compositions that may be used toproduce a desired hardness gradient, cores including such compositionsthat possess a positive hardness gradient, and golf balls including suchcores. In some respects, the tailorable hardness gradient in the coreprovides the ability to increase or reduce driver spin when compared toa conventional golf ball hit under the same conditions. In addition, thetailorable hardness gradient in the core may be used to provide greatercontrol on long shots as well as approach shots and greenside play.

While the golf ball core is functionally different from the other layersof the golf ball and operates somewhat independently, the core of thepresent disclosure greatly influences the overall performance of thefinished golf ball including such a core. Without being bound by anyparticular theory, performance characteristics of a finished golf ballthat contains the core of the present disclosure may be tailored bychanging the core composition. For example, altering the corecomposition and, thus, the hardness gradient, may have a significanteffect on long shots, e.g., shots off of a driver, and approach shots,e.g., shots made with irons and wedges. In fact, adjusting the hardnessgradient of cores made in accordance with this present disclosure, evenin relatively small amounts, can significantly affect how a golf ballperforms on long and short distance shots. Similarly, adjusting thehardness gradient of the core may allow for tailoring of otherproperties of the finished golf ball. The core formulations, cores, golfballs, and resulting performance characteristics are discussed ingreater detail below.

Core Formulations Formulation

A golf ball of the present disclosure may contain a single-ormulti-layered core. One or more of the layers of the core may comprise arubber formulation. In one embodiment, the rubber formulation includes abase rubber in an amount of about 5 percent to 100 percent by weightbased on the total weight of the rubber formulation. In one embodiment,the base rubber is included in the rubber formulation in an amountwithin a range having a lower limit of about 5 percent or 10 percent or20 percent or 30 percent or 40 percent or 50 percent or 55 percent andan upper limit of about 60 percent or 70 percent or 80 percent or 90percent or 95 percent or 100 percent. For example, the base rubber maybe present in the rubber formulation in an amount of about 30 percent toabout 80 percent by weight based on the total weight of the rubberformulation. In another example, the rubber formulation includes about40 percent to about 70 percent base rubber based on the total weight ofthe rubber formulation.

Concentrations are in parts per hundred (phr) unless otherwiseindicated. As used herein, the term, “parts per hundred,” also known as“phr” or “pph” is defined as the number of parts by weight of aparticular component present in a mixture, relative to 100 parts byweight of the polymer component. Mathematically, this can be expressedas the weight of an ingredient divided by the total weight of thepolymer, multiplied by a factor of 100.The base rubber may bepolybutadiene, polyisoprene, ethylene propylene rubber,ethylene-propylene-diene rubber, styrene-butadiene rubber, styrenicblock copolymer rubbers, polyalkenamers such as, for example,polyoctenamer, butyl rubber, halobutyl rubber, polystyrene elastomers,polyethylene elastomers, polyurethane elastomers, polyurea elastomers,metallocene-catalyzed elastomers and plastomers, copolymers ofisobutylene and p-alkylstyrene, halogenated copolymers of isobutyleneand p-alkylstyrene, copolymers of butadiene with acrylonitrile,polychloroprene, alkyl acrylate rubber, chlorinated isoprene rubber,acrylonitrile chlorinated isoprene rubber, and blends of two or morethereof. In one embodiment, the rubber formulation includespolybutadiene rubber, butyl rubber, or a blend thereof as the baserubber.

For example, the core may be formed from a rubber formulation thatincludes polybutadiene as the base rubber. Polybutadiene is ahomopolymer of 1,3-butadiene. The double bonds in the 1,3-butadienemonomer are attacked by catalysts to grow the polymer chain and form apolybutadiene polymer having a desired molecular weight. Any suitablecatalyst may be used to synthesize the polybutadiene rubber dependingupon the desired properties. In one embodiment, a transition metalcomplex (for example, neodymium, nickel, or cobalt) or an alkyl metalsuch as alkyl lithium is used as a catalyst. Other catalysts include,but are not limited to, aluminum, boron, lithium, titanium, andcombinations thereof. The catalysts produce polybutadiene rubbers havingdifferent chemical structures. In a cis-bond configuration, the maininternal polymer chain of the polybutadiene appears on the same side ofthe carbon-carbon double bond contained in the polybutadiene. In atrans-bond configuration, the main internal polymer chain is on oppositesides of the internal carbon-carbon double bond in the polybutadiene.The polybutadiene rubber can have various combinations of cis- andtrans-bond structures. For example, the polybutadiene rubber may have a1,4 cis-bond content of at least 40 percent. In another embodiment, thepolybutadiene rubber has a 1,4 cis-bond content of greater than 80percent. In still another embodiment, the polybutadiene rubber has a 1,4cis-bond content of greater than 90 percent. In general, polybutadienerubbers having a high 1,4 cis-bond content have high tensile strengthand rebound.

In some embodiments, the rubber formulation of the present disclosureincludes a blend of different polybutadiene rubbers. In this embodiment,the rubber formulation may include a blend of a first polybutadienerubber and a second polybutadiene rubber in a ratio of about 5:95 toabout 95:5. For example, the rubber formulation may include a firstpolybutadiene rubber and a second polybutadiene rubber in a ratio ofabout 10:90 to about 90:10 or about 15:85 to about 85:15 or about 20:80to about 80:20 or about 30:70 to about 70:30 or about 40:60 to about60:40. In other embodiments, the rubber formulation may include a blendof more than two polybutadiene rubbers or a blend of polybutadienerubber(s) with any of the other elastomers discussed above.

The polybutadiene rubber may have a relatively high or low Mooneyviscosity. Generally, polybutadiene rubbers of higher molecular weightand higher Mooney viscosity have better resiliency than polybutadienerubbers of lower molecular weight and lower Mooney viscosity. However,as the Mooney viscosity increases, the milling and processing of thepolybutadiene rubber generally becomes more difficult. Blends of highand low Mooney viscosity polybutadiene rubbers may be prepared as isdescribed in U.S. Pat. Nos. 6,982,301 and 6,774,187, the disclosures ofwhich are hereby incorporated by reference, and used in accordance withthis invention. In general, the lower limit of Mooney viscosity may beabout 30 or 35 or 40 or 45 or 50 or 55 or 60 or 70 or 75 and the upperlimit may be about 80 or 85 or 90 or 95 or 100 or 105 or 110 or 115 or120 or 125 or 130. For example, the polybutadiene used in the rubberformulation may have a Mooney viscosity of about 30 to about 80 or about40 to about 60.

Examples of commercially available polybutadiene rubbers that can beused in rubber formulations in accordance with this invention, include,but are not limited to, BR 01 and BR 1220, available from BST Elastomersof Bangkok, Thailand; SE BR 1220LA and SE BR1203, available from DOWChemical Co of Midland, Mich.; BUDENE 1207, 1207s, 1208, and 1280available from Goodyear, Inc of Akron, Ohio; BR 01, 51 and 730,available from Japan Synthetic Rubber (JSR) of Tokyo, Japan; BUNA CB 21,CB 22, CB 23, CB 24, CB 25, CB 29 MES, CB 60, CB Nd 60, CB 55 NF, CB 70B, CB KA 8967, and CB 1221, available from Lanxess Corp. of Pittsburgh.Pa.; BR1208, available from LG Chemical of Seoul, South Korea; UBEPOLBR130B, BR150, BR150B, BR150L, BR230, BR360L, BR710, and VCR617,available from UBE Industries, Ltd. of Tokyo, Japan; EUROPRENE NEOCIS BR60, INTENE 60 AF and P3OAF, and EUROPRENE BR HV80, available fromPolimeri Europa of Rome, Italy; AFDENE 50 and NEODENE BR40, BR45, BR50and BR60, available from Karbochem (PTY) Ltd. of Bruma, South Africa;KBR 01, NdBr 40, NdBR-45, NdBr 60, KBR 710S, KBR 710H, and KBR 750,available from Kumho Petrochemical Co., Ltd. Of Seoul, South Korea;DIENE 55NF, 70AC, and 320 AC, available from Firestone Polymers ofAkron, Ohio; and PBR-Nd Group II and Group III, available fromNizhnekamskneftekhim, Inc. of Nizhnekamsk, Tartarstan Republic.

In another embodiment, the core is formed from a rubber formulationincluding butyl rubber. Butyl rubber is an elastomeric copolymer ofisobutylene and isoprene. Butyl rubber is an amorphous, non-polarpolymer with good oxidative and thermal stability, good permanentflexibility and high moisture and gas resistance. Generally, butylrubber includes copolymers of about 70 percent to about 99.5 percent byweight of an isoolefin, which has about 4 to 7 carbon atoms, forexample, isobutylene, and about 0.5 percent to about 30 percent byweight of a conjugated multiolefin, which has about 4 to 14 carbonatoms, for example, isoprene. The resulting copolymer contains about 85percent to about 99.8 percent by weight of combined isoolefin and about0.2 percent to about 15 percent of combined multiolefin. A commerciallyavailable butyl rubber includes Bayer Butyl 301 manufactured by BayerAG.

In still another embodiment, the rubber formulation used to form thecore includes a blend of polybutadiene and butyl rubber. In thisembodiment, the rubber formulation may include a blend of polybutadieneand butyl rubber in a ratio of about 10:90 to about 90:10. For example,the rubber formulation may include a blend of polybutadiene and butylrubber in a ratio of about 10:90 to about 90:10 or about 20:80 to about80:20 or about 30:70 to about 70:30 or about 40:60 to about 60:40. Inother embodiments, the rubber formulation may include polybutadieneand/or butyl rubber in a blend with any of the other elastomersdiscussed above.

The rubber formulations of the present disclosure include a hardeningagent. Without being bound to any particular theory, the hardening agentmay affect the hardness of the core and the hardness gradient across thecore. Suitable hardening agents include, but are not limited to, benzoiccompounds comprising a nitro functional group and one of a hydroxyl,amino, or sulfhydryl functional group. Nonlimiting examples of hardeningagents include nitrophenol, nitroaniline, and nitrothiophenol. Differentisomers of the hardening agent may be used such as, for example,2-nitrophenol, 3-nitrophenol, 4-nitrophenol, 2-nitroaniline,3-nitroaniline, 4-nitroaniline, 2-nitrothiophenol, 3-nitrothiophenol,4-nitrothiophenol, and combinations thereof. Without being bound by anyparticular theory, different isomers of the hardening agent may affectthe hardness of the core differently and produce different hardnessgradients across the core. Some hardening agents, for examplenitrophenol, may be advantageous because they are safe and/or easy tohandle during manufacturing.

The hardening agent may be included in the rubber formulation in varyingamounts depending on the desired characteristics of the golf ball core.For example, the hardening agent may be used in an amount of 0.05 toabout 3 parts by weight per 100 parts of the total rubber. In oneembodiment, the rubber formulation of the core includes about 0.1 toabout 1.5 or about 0.1 to about 1 part by weight hardening agent per 100parts of the total rubber. In another embodiment, the hardening agent isincluded in the rubber formulation in an amount of about 0.2 to about0.7 parts by weight per 100 parts of the total rubber. In still anotherembodiment, the rubber formulation includes about 0.2 to about 0.4 orabout 0.3 to about 0.5 or about 0.4 to about 0.6 parts by weighthardening agent per 100 parts of the total rubber.

In some respects, the amount of hardening agent in the rubberformulation required to produce the desired hardness gradient may differbased on the compound, and even the particular isomer of the compound,used as the hardening agent. For example, when the rubber formulationincludes 2-nitrophenol, which has a nitro functional group ortho to ahydroxyl functional group, the hardening agent may be used in an amountof about 0.2 to about 0.4 parts by weight per 100 parts of the totalrubber to achieve the desired hardness gradient. In contrast, when therubber formulation includes either 3-nitrophenol, which has a nitrofunctional group meta to a hydroxyl functional group, or 4-nitrophenol,which has a nitro functional group para to a hydroxyl functional group,it may be useful to use about 0.4 to about 0.6 parts by weight hardeningagent per 100 parts of the total rubber to achieve the desired hardnessgradient. Without being bound by any particular theory, the relativepositions of the functional groups on disubstituted benzoic hardeningagents are believed to influence the effectiveness of the compound as ahardening agent. Accordingly, the amount of hardening agent needed toproduce a desired hardness gradient may change when different isomerswithin a class of compounds are used.

The rubber formulations further include a reactive cross-linkingco-agent. Suitable co-agents include, but are not limited to, metalsalts of unsaturated carboxylic acids having from 3 to 8 carbon atoms;unsaturated vinyl compounds and polyfunctional monomers (e.g.,trimethylolpropane trimethacrylate); phenylene bismaleimide; andcombinations thereof. In one embodiment, the co-agent is one or moremetal salts of acrylates, diacrylates, methacrylates, anddimethacrylates, wherein the metal is selected from magnesium, calcium,zinc, aluminum, lithium, and nickel. In another embodiment, the co-agentincludes one or more zinc salts of acrylates, diacrylates,methacrylates, and dimethacrylates. For example, the co-agent may bezinc diacrylate (ZDA). In another embodiment, the co-agent may be zincdimethacrylate (ZDMA). An example of a commercially available zincdiacrylate includes Dymalink® 526 manufactured by Cray Valley.

The co-agent may be included in the rubber formulation in varyingamounts depending on the desired characteristics of the golf ball core.For example, the co-agent may be used in an amount of about 25 to about50 parts by weight per 100 parts of the total rubber. In one embodiment,the rubber formulation of the core includes about 35 to about 48 partsby weight co-agent per 100 parts of the total rubber. In anotherembodiment, the rubber formulation includes about 38 to about 45 orabout 39 to about 42 parts by weight co-agent per 100 parts of totalrubber. In another embodiment, the co-agent is included in the rubberformulation of the core in an amount of about 29 to about 37 or about 31to about 35 parts by weight per 100 parts of the total rubber. In stillanother embodiment, the rubber formulation includes about 25 to about 33or about 27 to about 31 parts by weight co-agent per 100 parts of thetotal rubber.

In some respects, the amount of co-agent in the rubber formulation maybe altered based on the class of compounds, and the particular isomerwithin a class of compounds, used as the hardening agent. For example,when the rubber formulation includes 2-nitrophenol, the co-agent may beincluded in the rubber formulation in amount from about 37 to about 43or about 39 to about 41 parts by weight per 100 parts of the totalrubber. In another example, when the rubber formulation includes3-nitrophenol, the co-agent may be included in the rubber formulation inamount from about 30 to about 36 or about 32 to about 34 parts by weightper 100 parts of the total rubber. In yet another example, when therubber formulation includes 4-nitrophenol, the co-agent may be includedin the rubber formulation in amount from about 26 to about 32 or about28 to about 30 parts by weight per 100 parts of the total rubber.Without being bound to any particular theory, the concentration ofco-agent may be altered to achieve the desired compression of the golfball core when different hardening agent are used.

Radical scavengers such as a halogenated organosulfur, organicdisulfide, or inorganic disulfide compounds may also be added to therubber formulation. In one embodiment, a halogenated organosulfurcompound included in the rubber formulation includes, but is not limitedto, pentachlorothiophenol (PCTP) and salts of PCTP such as zincpentachlorothiophenol (ZnPCTP). In another embodiment, ditolyldisulfide, diphenyl disulfide, dixylyl disulfide, 2-nitroresorcinol, andcombinations thereof are added to the rubber formulation. An example ofa commercially available radical scavenger includes Rhenogran®Zn-PTCP-72 manufactured by Rheine Chemie. The radical scavenger may beincluded in the rubber formulation in an amount of about 0.3 to about 1part by weight per 100 parts of the total rubber. In one embodiment, therubber formulation may include about 0.4 to about 0.9 parts by weightradical scavenger per 100 parts of the total rubber. In anotherembodiment, the rubber formulation may include about 0.5 to about 0.8parts by weight radical scavenger per 100 parts of the total rubber.

The rubber formulation may also include filler(s). Suitable non-limitingexamples of fillers include carbon black, clay and nanoclay particles,talc, glass (e.g., glass flake, milled glass, and microglass), mica andmica-based pigments (e.g., Iriodin® pearl luster pigments from The MerckGroup), and combinations thereof. Metal oxide and metal sulfate fillersare also contemplated for inclusion in the rubber formulation. Suitablemetal fillers include, for example, particulate, powders, flakes, andfibers of copper, steel, brass, tungsten, titanium, aluminum, magnesium,molybdenum, cobalt, nickel, iron, lead, tin, zinc, barium, bismuth,bronze, silver, gold, and platinum, and alloys and combinations thereof.Suitable metal oxide fillers include, for example, zinc oxide, ironoxide, aluminum oxide, titanium oxide, magnesium oxide, and zirconiumoxide. Suitable metal sulfate fillers include, for example, bariumsulfate and strontium sulfate. When included, the fillers may be in anamount of about 1 to about 25 parts by weight per 100 parts of the totalrubber. In one embodiment, the rubber formulation includes at least onefiller in an amount of about 5 to about 20 or about 8 to about 15 partsby weight per 100 parts of the total rubber. In another embodiment, therubber formulation includes at least one filler in an amount of about 8to about 14 or about 10 to about 12 parts by weight per 100 parts of thetotal rubber. In yet another embodiment, the rubber formulation includesat least one filler in an amount of about 10 to about 17 or about 12 toabout 15 parts by weight per 100 parts of the total rubber. In yetanother embodiment, the rubber formulation includes at least one fillerin an amount of about 10 to about 16 or about 12 to about 15 parts byweight per 100 parts of the total rubber. In a further embodiment, therubber formulation includes at least one filler in an amount of about 12to about 18 or about 14 to about 16 parts by weight per 100 parts of thetotal rubber. An example of a commercially available barium sulfatefiller includes PolyWate® 325 manufactured by Cimbar PerformanceMinerals.

In some aspects, the amount of filler in the rubber formulation may bealtered based on the compound, and the particular isomer of thecompound, used as the hardening agent. For example, when the rubberformulation includes 2-nitrophenol, at least one filler may be includedin the rubber formulation in amount from about 9 to about 13 parts byweight per 100 parts of the total rubber. In another example, when therubber formulation includes 3-nitrophenol, the filler may be included inthe rubber formulation in amount from about 11 to about 16 parts byweight per 100 parts of the total rubber. In yet another example, whenthe rubber formulation includes 4-nitrophenol, the filler may beincluded in the rubber formulation in amount from about 13 to about 17parts by weight per 100 parts of the total rubber.

In some embodiments, more than one type of filler may be included in therubber formulation. For example, the rubber formulation may include afirst filler in an amount from about 5 to about 20 or about 8 to about17 parts by weight per 100 parts total rubber and a second filler in anamount from about 1 to about 10 or about 3 to about 7 parts by weightper 100 parts total rubber. In another example, the rubber formulationmay include a first filler in an amount from about 7 to about 13 orabout 9 to about 12 parts by weight per 100 parts total rubber and asecond filler in an amount from about 2 to about 8 or about 4 to about 6parts by weight per 100 parts total rubber. In yet another example, therubber formulation may include a first filler in an amount from about 10to about 15 or about 13 to about 14 parts by weight per 100 parts totalrubber and a second filler in an amount from about 2 to about 9 or about3 to about 7 parts by weight per 100 parts total rubber. In a furtherexample, the rubber formulation may include a first filler in an amountfrom about 10 to about 15 or about 13 to about 14 parts by weight per100 parts total rubber and a second filler in an amount from about 13 toabout 18 or about 14 to about 16 parts by weight per 100 parts totalrubber.

Antioxidants, processing aids, accelerators (for example, tetramethylthiuram), dyes and pigments, wetting agents, surfactants,plasticizers, coloring agents, fluorescent agents, chemical blowing andfoaming agents, defoaming agents, stabilizers, softening agents, impactmodifiers, antiozonants, as well as other additives known in the art,may also be added to the rubber formulation. Examples of suitableprocessing aids include, but are not limited to, high molecular weightorganic acids and salts thereof. Suitable organic acids are aliphaticorganic acids, aromatic organic acids, saturated mono-functional organicacids, unsaturated monofunctional organic acids, multi-unsaturatedmono-functional organic acids, and dimerized derivatives thereof. In oneembodiment, the organic acids include, but are not limited to, caproicacid, caprylic acid, capric acid, lauric acid, stearic acid, behenicacid, erucic acid, oleic acid, linoleic acid, myristic acid, benzoicacid, palmitic acid, phenylacetic acid, naphthalenoic acid, anddimerized derivatives thereof. The salts of organic acids include thesalts of barium, lithium, sodium, zinc, bismuth, chromium, cobalt,copper, potassium, strontium, titanium, tungsten, magnesium, cesium,iron, nickel, silver, aluminum, tin, or calcium, salts of fatty acids,particularly stearic, behenic, erucic, oleic, linoelic or dimerizedderivatives thereof.

The rubber formulation may be cured using conventional curing processes.Non-limiting examples of curing processes suitable for use in accordancewith the present disclosure include peroxide-curing, sulfur-curing,high-energy radiation, and combinations thereof. In one embodiment, therubber formulation includes a free-radical initiator selected fromorganic peroxides, high energy radiation sources capable of generatingfree-radicals, and combinations thereof. Suitable organic peroxidesinclude, but are not limited to, dicumyl peroxide;n-butyl-4,4-di(t-butylperoxy) valerate;1,1-di(t-butylperoxy)3,3,5-trimethylcyclohexane;2,5-dimethyl-2,5-di(t-butylperoxy) hexane; di-t-butyl peroxide;di-t-amyl peroxide; t-butyl peroxide; t-butyl cumyl peroxide;2,5-dimethyl-2,5-di(t-butylperoxy)hexyne-3;di(2-t-butyl-peroxyisopropyl)benzene; dilauroyl peroxide; dibenzoylperoxide; t-butyl hydroperoxide; and combinations thereof. In aparticular embodiment, the free radical initiator is dicumyl peroxide,including, but not limited to Perkadox® BD-FF, commercially availablefrom Akzo Nobel. Peroxide free-radical initiators may be present in therubber formulation in an amount of at least 0.05 parts by weight per 100parts of the total rubber, or an amount within the range having a lowerlimit of 0.05 parts or 0.1 parts or 1 part or 1.25 parts or 1.5 parts or2.5 parts or 5 parts by weight per 100 parts of the total rubber, and anupper limit of 2.5 parts or 3 parts or 5 parts or 6 parts or 10 parts or15 parts by weight per 100 parts of the total rubber. For example, therubber formulation may include peroxide free-radical initiators in anamount of about 0.1 to about 3.5 or about 0.5 to about 3 or about 1.3 toabout 2.2 parts by weight per 100 parts of the total rubber. In anotherexample, the rubber formulation may include peroxide free-radicalinitiators in an amount of about 0.7 to about 1.8 or about 0.8 to about1.2 or about 1.3 to about 1.7 parts by weight per 100 parts of the totalrubber. In yet another example, the rubber formulation may includeperoxide free-radical initiators in an amount of about 1.7 to about 2.8or about 1.8 to about 2.2 or about 2.3 to about 2.7 parts by weight per100 parts of the total rubber. In embodiments where a free-radicalinitiator is used, it may be desirable to combine the hardening agentinto the rubber formulation prior to adding the free-radical initiator.

Properties Hardness

The hardness of the geometric center of the core may be obtainedaccording to the following: the core is first gently pressed into ahemispherical holder having an internal diameter approximately slightlysmaller than the diameter of the core, such that the core is held inplace in the hemispherical portion of the holder while concurrentlyleaving the geometric central plane of the center exposed. The core issecured in the holder by friction, such that it will not move during thecutting and grinding steps, but the friction is not so excessive thatdistortion of the natural shape of the core would result. The core issecured such that the parting line of the center is roughly parallel tothe top of the holder. The diameter of the center is measured 90 degreesto this orientation prior to securing. A measurement is also made fromthe bottom of the holder to the top of the core to provide a referencepoint for future calculations. A rough cut is made slightly above theexposed geometric center of the core using a band saw or otherappropriate cutting tool, making sure that the core does not move in theholder during this step. The remainder of the core, still in the holder,is secured to the base plate of a surface grinding machine. The exposed‘rough’ surface is ground to a smooth, flat surface, revealing thegeometric center of the core, which can be verified by measuring theheight from the bottom of the holder to the exposed surface of the core,making sure that exactly half of the original height of the core, asmeasured above, has been removed to within 0.004 inches. Leaving thecore in the holder, the geometric center of the core is confirmed with acenter square and carefully marked, and the hardness is measured at thecenter mark according to ASTM D-2240.

Additional hardness measurements at any distance from the geometriccenter of the core can then be made by drawing a line radially outwardfrom the geometric center mark and measuring the hardness at any givendistance along the line, typically in 2 mm increments from the center ofthe core. The hardness at a particular distance from the geometriccenter should be measured along at least two, preferably four, radialarms located 180° apart, or 90° apart, respectively, and then averaged.All hardness measurements performed on a plane passing through thegeometric center are performed while the core is still in the holder andwithout having disturbed its orientation, such that the test surface isconstantly parallel to the bottom of the holder, and thus also parallelto the properly aligned foot of the durometer.

The outer surface hardness of the core (or any golf ball layer) ismeasured on the actual outer surface of the layer and is obtained fromthe average of a number of measurements taken from opposing hemispheres,taking care to avoid making measurements on the parting line of the coreor on surface defects, such as holes or protrusions and preferablymaking the measurements prior to surrounding the layer of interest withan additional layer. Hardness measurements are made pursuant to ASTMD-2240 “Indentation Hardness of Rubber and Plastic by Means of aDurometer.” Because of the curved surface, care must be taken to ensurethat the golf ball or golf ball sub-assembly is centered under thedurometer indenter before a surface hardness reading is obtained. Acalibrated, digital durometer, capable of reading to 0.1 hardness unitsis used for the hardness measurements. The digital durometer must beattached to, and its foot made parallel to, the base of an automaticstand. The weight on the durometer and attack rate conforms to ASTMD-2240. It is worthwhile to note that, once an additional layersurrounds a layer of interest, the hardness of the layer of interest canbe difficult to determine. Therefore, for purposes of the presentdisclosure, when the hardness of a layer is needed after the inner layerhas been surrounded with another layer, the test procedure for measuringa point located 1 mm from an interface is used.

It should also be noted that there is a fundamental difference between“material hardness” and “hardness as measured directly on a golf ball”(or, as used herein, “surface hardness”). For purposes of the presentdisclosure, material hardness is measured according to ASTM D2240 andgenerally involves measuring the hardness of a flat “slab” or “button”formed of the material. Surface hardness as measured directly on a golfball (or other spherical surface) typically results in a differenthardness value. The difference in “surface hardness” and “materialhardness” values is due to several factors including, but not limitedto, ball construction (that is, core type, number of layers, and thelike); ball (or ball sub-assembly) diameter; and the materialcomposition of adjacent layers. It also should be understood that thetwo measurement techniques are not linearly related and, therefore, onehardness value cannot easily be correlated to the other. Shore hardness(for example, Shore C or Shore D hardness) was measured according to thetest method ASTM D-2240.

A golf ball core made from the rubber formulation of the presentdisclosure may have a hardness at the geometric center of the core,referred to herein as H_(C), that ranges from about 40 to about 90 ShoreC. In one embodiment, the core has a hardness at its geometric center ofabout 45 to about 65 Shore C or about 48 to about 58 Shore C or about 49to about 52 Shore C. In another embodiment, the core has a hardness atits geometric center of about 55 to about 75 Shore C or about 60 toabout 66 Shore C or about 68 to about 74 Shore C. In yet anotherembodiment, the core has a hardness at its geometric center of about 65to about 85 Shore C or about 66 to about 74 Shore C or about 77 to about84 Shore C.

The hardness at the surface of the core, referred to herein as Hs, mayrange from about 60 to about 95 Shore C. In one embodiment, the hardnessat the surface of the core is about 70 to about 95 Shore C or about 72to about 82 Shore C or about 85 to about 95 Shore C or about 87 to about93 Shore C. In another embodiment, the hardness at the surface of thecore is about 65 to about 95 Shore C or about 73 to about 93 Shore C orabout 74 to about 84 Shore C. In yet another embodiment, the hardness atthe surface of the core is about 72 to about 95 Shore C or about 77 toabout 85 Shore C or about 88 to about 94 Shore C.

The direction of the hardness gradient is defined by the difference inhardness measurements taken at the geometric center and outer surfacesof the core. The geometric center hardness is readily determinedaccording to the test procedures provided above. For example, thehardness of the outer surface of the core is also readily determinedaccording to the procedures given herein for measuring the outer surfacehardness of a golf ball layer, if the measurement is made prior tosurrounding the core with additional layers.

While the hardness gradient across the core will vary based on severalfactors including, but not limited to, the dimensions and formulationsof the components, the core of the present disclosure has a “positive”hardness gradient (that is, the geometric center is softer than theouter surface of the core). More particularly, the term, “positivehardness gradient” as used herein means a hardness gradient of positiveabout 2 Shore C or greater, about 4 Shore C or greater, about 6 Shore Cor greater, about 8 Shore C or greater, or about 10 Shore C or greater.In general, the hardness gradient may be determined by subtracting thehardness value of one component being measured (for example, thegeometric center of the core, H_(C)) from the hardness value of anothercomponent being measured (for example, the outer surface of the core,H_(S)).

The core of the present disclosure has a positive hardness gradient. Inone embodiment, the core has a positive hardness gradient from thegeometric center to the surface of the core of about 2 Shore C to 42Shore C. In this aspect, the positive hardness gradient of the core isabout 5 Shore C to about 40 Shore C. The rubber formulation of the coremay be tailored to produce a desired hardness gradient in the core. Insome embodiments, the positive hardness gradient of the core is about 30to about 42 Shore C or about 34 Shore C to 41 Shore C or about 37 ShoreC to about 40 Shore C. In other embodiments, the positive hardnessgradient of the core is about 3 Shore C to about 25 Shore C or about 10Shore C to about 23 Shore C, or about 11 Shore C to about 17 Shore C. Infurther embodiments, the positive hardness gradient of the core may beabout 2 Shore C to about 40 Shore C or about 7 Shore C to about 12 ShoreC or about 8 Shore C to 11 Shore C.

The hardness of the core may not increase linearly from the center ofthe core to the outer surface of the core. For example, one or moreregions within the core may have a “zero” hardness gradient, i.e., thehardness values across the region are substantially the same. The term,“zero hardness gradient” as used herein means a hardness gradient of −2Shore C to 2 Shore C, preferably between about −1 Shore C and about 1Shore C and may have a value of zero. In some embodiments, one or moreregions of the core may also have a “negative” hardness gradient, i.e.,the hardness values across the region may decrease from the inner edgeof the region to the outer edge of the region.

For example, the core, or a layer of the core if the core has multiplelayers, may be characterized by three regions: an inner region, anintermediate region, and an outer region. Each of the inner region,intermediate region, and outer region may have its own hardnessgradient. For a single-layer core, the inner region is the region of thecore surrounding the center of the core and is characterized by positivehardness gradient of about 2 Shore C to about 25 Shore C. In someembodiments, the positive hardness gradient of the inner region of thecore is about 6 Shore C to about 25 Shore C or about 16 Shore C to about23 Shore C. In other embodiments, the positive hardness gradient of theinner region of the core is about 1 Shore C to about 13 Shore C or about6 Shore C to about 11 Shore C. In further embodiments, the positivehardness gradient of the inner region of the core is about 5 Shore C toabout 9 Shore C or about 6 Shore C to about 8 Shore C.

The outer region of the core is the region of the core adjacent thesurface of the core and may be characterized by a zero or positivehardness gradient from about −2 Shore C to about 28 Shore C. In someembodiments, the outer region may have a positive hardness gradient from2 Shore C to about 27 Shore C or about 16 Shore C to about 27 Shore C orabout 17 Shore C to about 22 Shore C. In other embodiments, the outerregion may have a zero or positive hardness gradient from −2 Shore C toabout 16 Shore C or about 2 Shore C to about 6 Shore C or about 10 ShoreC to about 15 Shore C. In further embodiments, the outer region may havea zero or positive hardness gradient from −2 Shore C to about 14 Shore Cor about 1 Shore C to about 8 Shore C or about 2 Shore C to about 6Shore C.

The intermediate region of the core is the region of the core betweenthe inner region and the outer region and may be characterized by anegative, zero, or positive hardness gradient from about −10 to 8 ShoreC. In some embodiments, the intermediate region may have a negative,zero, or positive hardness gradient from −7 to about 6 Shore C or about−6 to about 1 Shore C. In other embodiments, the intermediate region mayhave a positive hardness gradient from −7 to about 4 Shore C or about −2to about 4 Shore C. In further embodiments, the intermediate region mayhave a negative or zero hardness gradient from −10 to about 0 Shore C orabout −4 Shore C to about 0 Shore C.

In some embodiments, a point or plurality of points measured along a“positive” gradient may be above or below a line fit through thegradient and its outermost and innermost hardness values. In analternative embodiment, the hardest point along a particular steep“positive” gradient may be higher than the value at the innermostportion of the center (the geometric center) or outer surface of thecore—as long as the outermost point (i.e., the outer surface of thecore) is greater than the innermost point (i.e., the geometric center ofthe core), such that the “positive” gradients remain intact.

Compression

Several different methods can be used to measure compression, includingAtti compression, Riehle compression, load/deflection measurements at avariety of fixed loads and offsets, and effective modulus (see, e.g.,Compression by Any Other Name, Science and Golf IV, Proceedings of theWorld Scientific Congress of Golf (Eric Thain ed., Routledge, 2002) (J.Dalton). For purposes of the present disclosure, compression values areprovided as measured by the Dynamic Compression Machine (“DCM”) as wellas the Soft Center Deflection Index (“SCDI”). The DCM applies a load toa ball component or a ball and measures the number of inches the core orball is deflected at measured loads. A crude load/deflection curve isgenerated that is fit to the Atti compression scale that results in anumber being generated that represents an Atti compression. The DCM doesthis via a load cell attached to the bottom of a hydraulic cylinder thatis triggered pneumatically at a fixed rate (typically about 1.0 ft/s)towards a stationary core. Attached to the cylinder is an LVDT thatmeasures the distance the cylinder travels during the testing timeframe.A software-based logarithmic algorithm ensures that measurements are nottaken until at least five successive increases in load are detectedduring the initial phase of the test.

The SCDI is a slight variation of the DCM set up that allowsdetermination of the pounds required to deflect a component or ball 10percent of its diameter. With the SCDI, the goal is to obtain the poundsof force required to deflect a component or ball a certain number ofinches. That amount of deflection is 10 percent of the component or balldiameter. The DCM is triggered, the cylinder deflects the component orball by 10 percent of its diameter, and the DCM reports back the poundsof force required (as measured from the attached load cell) to deflectthe component or ball by that amount. The SCDI value obtained is asingle number in units of pounds.

The compression of a core made from the rubber formulation of thepresent disclosure may range from about 20 to about 120 DCM or morepreferably about 50 to about 120 DCM. For example, the core compressionmay be about 50 to about 85 DCM or about 60 to 80 DCM or about 65 toabout 75 DCM. In another example, the core compression may range fromabout 50 to about 100 DCM or about 55 to about 65 DCM or about 80 to 100DCM. In yet another example, the core compression is about 60 to about120 DCM or about 110 to about 120 DCM or about 60 to about 80 DCM orabout 71 to about 79 DCM. In some embodiments, it may be desirable for acore comprising the rubber formulation of the present disclosure to havea compression from about 68 to about 75 DCM or from about 70 to about 74DCM regardless of the hardening agent used.

Diameter

The diameter of the core may vary. In some embodiments, the corediameter may range from about 1.5 to about 1.58 inches. For example, thecore may have a diameter of 1.53 to 1.56 inches. In embodiments wherethe core comprises two or more layers, the diameter of the inner layerof the core may range from about 1.0 to about 1.4 inches or from about1.0 to about 1.2 inches.

Core Component Relationships

In one embodiment, the amount of hardening agent and the isomer ofhardening agent present in the rubber formulation used to form the coreare related to the hardness gradient of the core, according to therelationship shown in Equation I below:

$\begin{matrix}{\frac{HA_{C}*HA_{LN}}{1 - \frac{1}{H_{Gr}}} \geq {0.2}} & (I)\end{matrix}$where HA_(C) represents the concentration of hardening agent in therubber formulation in parts per hundred; H_(Gr) represents the hardnessgradient (Shore C) of the core or H_(S)−H_(C), and H_(Gr)≥2; and HA_(LN)represents the isomer number, i.e., the location number of the secondfunctional group on the hardening agent. For example, if the hardeningagent is 2-nitrophenol, HA_(LN) is equal to 2, and if the hardeningagent is 4-nitrophenol, HA_(LN) is equal to 4. In another embodiment,

$\frac{HA_{C}*HA_{LN}}{1 - \frac{1}{H_{Gr}}} \geq {0.3}$In still another embodiment,

$\frac{HA_{C}*HA_{LN}}{1 - \frac{1}{H_{Gr}}} \geq {0.4}$In some aspects,

${0.6} < \frac{HA_{C}*HA_{LN}}{1 - \frac{1}{H_{Gr}}} < {{0.6}3}$In other aspects

${1.5} < \frac{HA_{C}*HA_{LN}}{1 - \frac{1}{H_{G\tau}}} < {1.8}$In yet other aspects,

${2.0} < \frac{HA_{C}*HA_{LN}}{1 - \frac{1}{H_{G\tau}}} < {2.5}$

In another aspect, the isomer of hardening agent present in the rubberformulation used to form the core is related to the hardness gradient(Shore C) of the core according to the relationship shown in Equation IIbelow:

$\begin{matrix}{\frac{{HA}_{LN}}{1 - \frac{1}{H_{Gr}}} > 2} & ({II})\end{matrix}$where H_(Gr) represents the hardness gradient (Shore C) of the core orH_(S)−H_(C), and H_(Gr)≥2; and HA_(LN) represents the location number ofthe second functional group on hardening agent. In another embodiment,

$2 < \frac{{HA}_{LN}}{1 - \frac{1}{H_{Gr}}} < {2.2}$In still another embodiment,

$3 < \frac{{HA}_{LN}}{1 - \frac{1}{H_{Gr}}} < {3.7}$In yet another embodiment,

${4.2} < \frac{{HA}_{LN}}{1 - \frac{1}{H_{Gr}}} < {4.8}$

In another embodiment, the amount of hardening agent, the isomer of thehardening agent, and the amount of initiator present in the rubberformulation used to form the core are related to the hardness gradientof the core, according to the relationship shown in Equation III below:

$\begin{matrix}{\frac{HA_{C}*HA_{LN}}{1 - \frac{I_{C}}{H_{Gr}}} \geq {0.6}} & ({III})\end{matrix}$where HA_(C) represents the concentration of hardening agent in therubber formulation in parts per hundred; H_(Gr) represents the hardnessgradient (Shore C) of the core, and H_(Gr)≥2; HA_(LN) represents thelocation number of the second functional group on the hardening agent;and I_(C) represents the concentration of the initiator in the rubberformulation in parts per hundred. In another embodiment,

${{0.6}2} < \frac{{HA}_{C}*{HA}_{LN}}{1 - \frac{I_{C}}{H_{Gr}}} < {{0.6}4}$In still another embodiment,

${1.6} < \frac{{HA}_{C}*{HA}_{LN}}{1 - \frac{I_{c}}{H_{Gr}}} < {2.0}$In still another embodiment,

${2.4} < \frac{{HA}_{C}*{HA}_{LN}}{1 - \frac{I_{C}}{H_{Gr}}} < {2.7}$

In another aspect, the amount of hardening agent and the isomer of thehardening agent present in the rubber formulation used to form the coreare related to the hardness gradient of the core, according to therelationship shown in Equation IV below:

$\begin{matrix}{\frac{{HA}_{C}*{HA}_{LN}}{H_{Gr}^{0.2}} \geq {{0.2}5}} & ({IV})\end{matrix}$where HA_(C) represents the concentration of hardening agent in therubber formulation in parts per hundred; H_(Gr) represents the hardnessgradient (Shore C) of the core, and H_(Gr)≥2; and HA_(LN) represents thelocation number of the second functional group on the hardening agent.In another embodiment,

${{0.2}5} \leq \frac{{HA}_{C}*{HA}_{LN}}{H_{Gr}^{0.2}} \leq {{0.3}5}$In still another embodiment,

${{0.8}0} \leq \frac{{HA}_{C}*{HA}_{LN}}{H_{Gr}^{0.2}} \leq {0.9}$In still another embodiment,

${{1.2}0} \leq \frac{{HA}_{C}*{HA}_{LN}}{H_{Gr}^{0.2}} \leq {1.4}$

Golf Ball Construction

Golf balls having various constructions may be made in accordance withthis invention. For example, golf balls having one-piece, two-piece,three-piece, four-piece, and five or more-piece constructions with theterm “piece” refer to any core, cover, or intermediate layer of a golfball construction. Representative illustrations of such golf ballconstructions are provided and discussed further below. The term,“layer” as used herein means generally any spherical portion of the golfball.

In one embodiment, a golf ball of the present disclosure is a one-pieceball where the core and cover form a single integral layer. In anotherversion, shown in FIG. 1 , a golf ball of the present disclosure is atwo-piece ball 10 comprising a single core layer 12 and a single coverlayer 14. As shown in FIG. 2 , in one embodiment, the golf ball 20comprises a core layer 22, an intermediate layer 24, and a cover layer26. In FIG. 2 , the intermediate layer 24 can be considered an outercore layer, an inner cover layer, a mantle or casing layer, or any otherlayer disposed between the core 22 and the cover layer 26. Referring toFIG. 3 , in another embodiment, a four-piece golf ball 30 comprises aninner core layer 32, an outer core layer 34, an intermediate layer 36,and an outer cover layer 38. In FIG. 3 , the intermediate layer 36 maybe considered a casing or mantle layer, or inner cover layer, or anyother layer disposed between the outer core layer 34 and the outer coverof the ball 38. Referring to FIG. 4 , in another version, a five-piecegolf ball 40 comprises a three-layered core having an inner core layer42, an intermediate core layer 44, an outer core layer 46, an innercover layer 48, and an outer cover layer 50. As exemplified herein, agolf ball in accordance with the present disclosure can comprise anycombination of any number of core layers, intermediate layers, and coverlayers.

The rubber formulations discussed above are suitable for use in the coreor one or more of the core layers if multiple core layers are present.It is also contemplated that the rubber formulations disclosed hereinmay be used to form one or more of the layers of any of the one, two,three, four, or five, or more-piece (layered) balls described above.That is, any of the core layers, intermediate layers, and/or coverlayers may comprise the rubber formulation of this disclosure. Therubber formulations of different layers may be the same or different.The diameter and thickness of the different layers along with propertiessuch as hardness and compression may vary depending upon theconstruction and desired playing performance properties of the golfball.

Golf balls made in accordance with this invention can be of any size,although the USGA requires that golf balls used in competition have adiameter of at least 1.68 inches. For play outside of United States GolfAssociation (USGA) rules, the golf balls can be of a smaller size. Inone embodiment, golf balls made in accordance with this invention have adiameter in the range of about 1.68 to about 1.80 inches.

In contrast to the core, the cover of a golf ball plays less of a roleon shots off of a driver. However, because the cover plays a large rolein generating spin on iron and wedge shots, the cover material andproperties are still important. In this aspect, different materials maybe used in the construction of the intermediate and cover layers of golfballs according to the present disclosure. For example, a variety ofmaterials may be used for forming the outer cover including, forexample, polyurethanes; polyureas; copolymers, blends and hybrids ofpolyurethane and polyurea; olefin-based copolymer ionomer resins;polyethylene, including, for example, low density polyethylene, linearlow density polyethylene, and high density polyethylene; polypropylene;rubber-toughened olefin polymers; acid copolymers, for example,poly(meth)acrylic acid, which do not become part of an ionomericcopolymer; plastomers; flexomers; styrene/butadiene/styrene blockcopolymers; styrene/ethylene-butylene/styrene block copolymers;dynamically vulcanized elastomers; copolymers of ethylene and vinylacetates; copolymers of ethylene and methyl acrylates; polyvinylchloride resins; polyamides, poly(amide-ester) elastomers, and graftcopolymers of ionomer; cross-linked trans-polyisoprene and blendsthereof; polyester-based thermoplastic elastomers; polyurethane-basedthermoplastic elastomers; synthetic or natural vulcanized rubber; andcombinations thereof.

In one embodiment, the cover is formed from a polyurethane, polyurea, orhybrid of polyurethane-polyurea. When used as cover layer materials,polyurethanes and polyureas can be thermoset or thermoplastic. Thermosetmaterials can be formed into golf ball layers by conventional casting orreaction injection molding techniques. Thermoplastic materials can beformed into golf ball layers by conventional compression or injectionmolding techniques.

Conventional and non-conventional materials may be used for formingintermediate layers of the ball including, for instance, ionomer resins,highly neutralized polymers, polybutadiene, butyl rubber, and otherrubber-based core formulations, and the like. In one embodiment, theinner cover layer, i.e., the layer disposed between the core and theouter cover, includes an ionomer. In this aspect, ionomers suitable foruse in accordance with the present disclosure may include partiallyneutralized ionomers and highly neutralized ionomers (HNPs), includingionomers formed from blends of two or more partially-neutralizedionomers, blends of two or more highly-neutralized ionomers, and blendsof one or more partially-neutralized ionomers with one or morehighly-neutralized ionomers. For purposes of the present disclosure,“HNP” refers to an acid copolymer after at least 70 percent of all acidgroups present in the composition are neutralized.

Preferred ionomers are salts of O/X- and O/X/Y-type acid copolymers,wherein O is an α-olefin, X is a C₃-C₈ α, β-ethylenically unsaturatedcarboxylic acid, and Y is a softening monomer. O is preferably selectedfrom ethylene and propylene. X is preferably selected from methacrylicacid, acrylic acid, ethacrylic acid, crotonic acid, and itaconic acid.Methacrylic acid and acrylic acid are particularly preferred. Y ispreferably selected from (meth) acrylate and alkyl (meth) acrylateswherein the alkyl groups have from 1 to 8 carbon atoms, including, butnot limited to, n-butyl (meth) acrylate, isobutyl (meth) acrylate,methyl (meth) acrylate, and ethyl (meth) acrylate.

Preferred O/X and O/X/Y-type copolymers include, without limitation,ethylene acid copolymers, such as ethylene/(meth)acrylic acid,ethylene/(meth)acrylic acid/maleic anhydride, ethylene/(meth)acrylicacid/maleic acid mono-ester, ethylene/maleic acid, ethylene/maleic acidmono-ester, ethylene/(meth)acrylic acid/n-butyl (meth)acrylate,ethylene/(meth)acrylic acid/iso-butyl (meth)acrylate,ethylene/(meth)acrylic acid/methyl (meth)acrylate,ethylene/(meth)acrylic acid/ethyl (meth)acrylate terpolymers, and thelike. The term, “copolymer,” as used herein, includes polymers havingtwo types of monomers, those having three types of monomers, and thosehaving more than three types of monomers. Preferred α, B-ethylenicallyunsaturated mono- or dicarboxylic acids are (meth) acrylic acid,ethacrylic acid, maleic acid, crotonic acid, fumaric acid, itaconicacid. (Meth) acrylic acid is most preferred. As used herein, “(meth)acrylic acid” means methacrylic acid and/or acrylic acid. Likewise,“(meth) acrylate” means methacrylate and/or acrylate.

In a particularly preferred version, highly neutralized E/X- andE/X/Y-type acid copolymers, wherein E is ethylene, X is a C₃-C₈ α,β-ethylenically unsaturated carboxylic acid, and Y is a softeningmonomer are used. X is preferably selected from methacrylic acid,acrylic acid, ethacrylic acid, crotonic acid, and itaconic acid.Methacrylic acid and acrylic acid are particularly preferred. Y ispreferably an acrylate selected from alkyl acrylates and aryl acrylatesand preferably selected from (meth) acrylate and alkyl (meth) acrylateswherein the alkyl groups have from 1 to 8 carbon atoms, including, butnot limited to, n-butyl (meth) acrylate, isobutyl (meth) acrylate,methyl (meth) acrylate, and ethyl (meth) acrylate. Preferred E/X/Y-typecopolymers are those wherein X is (meth) acrylic acid and/or Y isselected from (meth) acrylate, n-butyl (meth) acrylate, isobutyl (meth)acrylate, methyl (meth) acrylate, and ethyl (meth) acrylate. Morepreferred E/X/Y-type copolymers are ethylene/(meth) acrylic acid/n-butylacrylate, ethylene/(meth) acrylic acid/methyl acrylate, andethylene/(meth) acrylic acid/ethyl acrylate.

The amount of ethylene in the acid copolymer may be at least about 15weight percent, at least about 25 weight percent, at least about 40weight percent, or at least about 60 weight percent, based on totalweight of the copolymer. The amount of C₃ to C₈ α, β-ethylenicallyunsaturated mono- or dicarboxylic acid in the acid copolymer istypically from 1 weight percent to 35 weight percent, from 5 weightpercent to 30 weight percent, from 5 weight percent to 25 weightpercent, or from 10 weight percent to 20 weight percent, based on totalweight of the copolymer. The amount of optional softening comonomer inthe acid copolymer may be from 0 weight percent to 50 weight percent,from 5 weight percent to 40 weight percent, from 10 weight percent to 35weight percent, or from 20 weight percent to 30 weight percent, based ontotal weight of the copolymer.

The various O/X, E/X, O/X/Y, and E/X/Y-type copolymers are at leastpartially neutralized with a cation source, optionally in the presenceof a high molecular weight organic acid, such as those disclosed in U.S.Pat. No. 6,756,436, the entire disclosure of which is herebyincorporated herein by reference. The acid copolymer can be reacted withthe optional high molecular weight organic acid and the cation sourcesimultaneously, or prior to the addition of the cation source. Suitablecation sources include, but are not limited to, metal ion sources, suchas compounds of alkali metals, alkaline earth metals, transition metals,and rare earth elements; ammonium salts and monoamine salts; andcombinations thereof. Preferred cation sources are compounds ofmagnesium, sodium, potassium, cesium, calcium, barium, manganese,copper, zinc, lead, tin, aluminum, nickel, chromium, lithium, and rareearth metals. The amount of cation used in the composition is readilydetermined based on desired level of neutralization. As discussed above,for HNP compositions, the acid groups are neutralized to 70 percent orgreater, 70 to 100 percent, or 90 to 100 percent. In one embodiment, anexcess amount of neutralizing agent, that is, an amount greater than thestoichiometric amount needed to neutralize the acid groups, may be used.That is, the acid groups may be neutralized to 100 percent or greater,for example 110 percent or 120 percent or greater. In other embodiments,partially neutralized compositions are prepared, wherein 10 percent orgreater, normally 30 percent or greater of the acid groups areneutralized. When aluminum is used as the cation source, it ispreferably used at low levels with another cation such as zinc, sodium,or lithium, since aluminum has a dramatic effect on melt flow reductionand cannot be used alone at high levels. For example, aluminum is usedto neutralize about 10 percent of the acid groups and sodium is added toneutralize an additional 90 percent of the acid groups.

“Low acid” and “high acid” ionomeric polymers, as well as blends of suchionomers, may be used. In general, low acid ionomers are considered tobe those containing 16 weight percent or less of acid moieties, whereashigh acid ionomers are considered to be those containing greater than 16weight percent of acid moieties. In one embodiment, the inner coverlayer is formed from a composition comprising a high acid ionomer. Asuitable high acid ionomer is Surlyn® 8150. (Dow), which is a copolymerof ethylene and methacrylic acid, having an acid content of 19 weightpercent, 45 percent neutralized with sodium. In another embodiment, theinner cover layer is formed from a composition comprising a high acidionomer and a maleic anhydride-grafted non-ionomeric polymer. An exampleof a suitable maleic anhydride-grafted polymer is Fusabond® 525D (Dow),which is a maleic anhydride-grafted, metallocene-catalyzedethylene-butene copolymer having about 0.9 weight percent maleicanhydride grafted onto the copolymer. Blends of high acid ionomers withmaleic anhydride-grafted polymers are further disclosed, for example, inU.S. Pat. Nos. 6,992,135 and 6,677,401, the entire disclosures of whichare hereby incorporated herein by reference.

The inner cover layer also may be formed from a composition comprising a50/45/5 blend of Surlyn® 8940/Surlyn® 9650/Nucrel® 960. In this aspect,the composition may have a material hardness of from 80 to 85 Shore C.In another embodiment, the inner cover layer is formed from acomposition comprising a 50/25/25 blend of Surlyn® 8940/Surlyn®9650/Surlyn® 9910, having a material hardness of about 85 to 95 Shore C.In yet another embodiment, the inner cover layer is formed from acomposition comprising a 50/50 blend of Surlyn® 8940/Surlyn® 9650,having a material hardness of about 82 to 90 Shore C. A compositioncomprising a 50/50 blend of Surlyn® 8940 and Surlyn® 7940 also may beused.

The compositions used to make the layers outside of the core, e.g., theouter cover layer and, when present, the inner cover layer, may containa variety of fillers and additives to impart specific properties to theball. For example, relatively heavy-weight and light-weight metalfillers such as, particulate; powders; flakes; and fibers of copper,steel, brass, tungsten, titanium, aluminum, magnesium, molybdenum,cobalt, nickel, iron, lead, tin, zinc, barium, bismuth, bronze, silver,gold, and platinum, and alloys and combinations thereof may be used toadjust the specific gravity of the ball. Other additives and fillersinclude, but are not limited to, optical brighteners, coloring agents,fluorescent agents, whitening agents, UV absorbers, light stabilizers,surfactants, processing aids, antioxidants, stabilizers, softeningagents, fragrance components, plasticizers, impact modifiers, titaniumdioxide, clay, mica, talc, glass flakes, milled glass, and mixturesthereof.

The outer cover layer preferably has a material hardness of 85 Shore Cor less. The thickness of the outer cover layer is preferably within arange having a lower limit of 0.010 or 0.015 or 0.025 inches and anupper limit of 0.035 or 0.040 or 0.055 or 0.080 inches. Methods formeasuring hardness of the layers in the golf ball are described infurther detail above. When included, the inner cover layer preferablyhas a material hardness within a range having a lower limit of 70 or 75or 80 or 82 Shore C and an upper limit of 85 or 86 or 90 or 92 Shore C.The thickness of the intermediate layer is preferably within a rangehaving a lower limit of 0.010 or 0.015 or 0.020 or 0.030 inches and anupper limit of 0.035 or 0.045 or 0.080 or 0.120 inches.

In one embodiment, the golf balls made in accordance with the presentdisclosure include a core comprising a rubber formulation as describedherein, an inner cover layer formed from an ionomeric material, and theouter cover layer is formed from a polyurethane material, and the outercover layer has a hardness that is less than that of the inner coverlayer. For example, the inner cover layer may have a hardness of greaterthan about 60 Shore D and the outer cover layer may have a hardness ofless than about 60 Shore D. In an alternative embodiment, the innercover layer is comprised of a partially or fully neutralized ionomer, athermoplastic polyester elastomer, a thermoplastic polyether blockamide, or a thermoplastic or thermosetting polyurethane or polyurea, andthe outer cover layer is comprised of an ionomeric material. In thisalternative embodiment, the inner cover layer may have a hardness ofless than about 60 Shore D and the outer cover layer may have a hardnessof greater than about 55 Shore D and the inner cover layer hardness isless than the outer cover layer hardness.

When a dual cover is disposed about the core, the inner cover layer mayhave a thickness of about 0.01 inches to about 0.06 inches, about 0.015inches to about 0.040 inches, or about 0.02 inches to about 0.035inches. The outer cover layer may have a thickness of about 0.015 inchesto about 0.055 inches, about 0.02 inches to about 0.04 inches, or about0.025 inches to about 0.035 inches.

The golf balls of the present disclosure may be formed using a varietyof application techniques. For example, the golf ball, golf ball core,or any layer of the golf ball may be formed using compression molding,flip molding, injection molding, retractable pin injection molding,reaction injection molding (RIM), liquid injection molding (LIM),casting, vacuum forming, powder coating, flow coating, spin coating,dipping, spraying, and the like. Conventionally, compression molding andinjection molding are applied to thermoplastic materials, whereas RIM,liquid injection molding, and casting are employed on thermosetmaterials. In this aspect, cover layers may be formed over the coreusing any suitable technique that is associated with the material usedto form the layer. Preferably, each cover layer is separately formedover the core. For example, an ethylene acid copolymer ionomercomposition may be injection-molded to produce half-shells over thecore. Alternatively, the ionomer composition can be placed into acompression mold and molded under sufficient pressure, temperature, andtime to produce the hemispherical shells, which may then be placedaround the core in a compression mold. An outer cover layer including apolyurethane or polyurea composition over the ball sub-assembly may beformed by using a casting process.

Golf balls made in accordance with the present disclosure may besubjected to finishing steps such as flash-trimming, surface-treatment,marking, coating, and the like using techniques known in the art. In oneembodiment, a white-pigmented cover may be surface-treated using asuitable method such as, for example, corona, plasma, or ultraviolet(UV) light-treatment. Indicia such as trademarks, symbols, logos,letters, and the like may be printed on the cover using pad-printing,ink-jet printing, dye-sublimation, or other suitable printing methods.Clear surface coatings (for example, primer and topcoats), which maycontain a fluorescent whitening agent, may be applied to the cover. Golfballs may also be painted with one or more paint coatings in a varietyof colors. In one embodiment, white primer paint is applied first to thesurface of the ball and then a white top-coat of paint may be appliedover the primer.

Aerodynamic Characteristics

Golf ball spin rate is the amount of spin on the golf ball once the ballis hit and separates from the clubface of the golf club. Spin rate ismeasured by RPM (revolutions per minute). As described above, the golfballs of the present disclosure combine a core with a positive hardnessgradient with a cover to tailor the spin rate to produce a desiredperformance on shots off of a driver and on approach shots when comparedto a “conventional golf ball”. For the purposes of this disclosure, the“conventional golf ball” used for comparison purposes includes athree-piece golf ball with a polybutadiene core, an inner cover layerformed from ionomer, and a polyurethane outer cover. In particular, forthis spin rate comparison exercise, the inner cover and outer cover ofthe conventional golf ball are the same or substantially the same as theinner and outer cover layers of golf balls of the present disclosure.Additionally, both the conventional golf ball and golf balls of thepresent disclosure comprised a single core layer. In other words, theonly variation between the conventional golf ball and the golf ballsmade according to the present disclosure (for the purposes of this spinrate comparison exercise) was the rubber formulation of the core.

For example, if the average spin rate of a conventional golf ball off ofa driver is about 2700 rpm, the spin rate of the golf balls of thepresent disclosure are about the same or less by about 10 rpm to about150 rpm, i.e., about 2550 to about 2690 rpm (all other factors heldconstant). In some embodiments, the spin rate of the golf balls of thepresent disclosure are about 20 rpm to about 100 rpm less than the spinrate of a conventional golf ball off of a driver (all other factors heldconstant).

In one embodiment, the driver spin rate of a golf ball made according tothe present disclosure ranges from about 2000 rpm to about 3500 rpm. Inanother embodiment, the driver spin rate is about 2200 rpm to about 3000rpm. In yet another embodiment, the driver spin rate is about 2550 toabout 2800 rpm.

EXAMPLES

The invention is further illustrated by the following examples. Itshould be understood that the examples below are for illustrativepurposes only. These examples should not be construed as limiting thescope of the invention.

The golf ball cores and golf balls of these examples are made usingrubber formulations in accordance with the present disclosure. Thecompositions of the rubber formulations of each golf ball core includedin the examples below is provided. These examples provide the componentsincluded in each rubber formulation. Concentrations of each componentare provided in parts by weight per 100 parts of base rubber unlessstated otherwise. Further, data on select characteristics, includinghardness, hardness gradient, compression, COR, core weight, corediameter, and, in some cases, spin data, was collected for the golf ballcores made from the compositions. In the examples below, the hardnessgradient refers the difference in the hardness at the surface and thehardness at the geometric center of the golf ball core below.

Example 1

The following examples describe golf ball cores having a single layer(solid sphere) made from rubber formulations in accordance with thepresent invention. The rubber formulations below include a base rubbercomprised of a bend of polybutadiene rubbers, Buna CB 1221 and Buna® CB24; a co-agent, Dymalink® 526 (zinc diacrylate); a first filler, zincoxide (ZnO); a second filler, PolyWate® 325 (barium sulfate); a radicalscavenger, Rhenogran® Zn-PCTP-72; an initiator, Perkadox® BD-FF (dicumylperoxide); and a hardening agent, 2-nitrophenol. The compositions andproperties of the cores are described in Table 1 below.

TABLE 1 Golf Ball Cores Including 2-Nitrophenol A B C D CompositionBuna ® CB 1221 85 85 85 85 Buna ® CB 24 15 15 15 15 Zinc Oxide 5 5 5 5PolyWate ® 325 10.8 10.8 10.7 10.8 Rhenogran ® 0.7 0.7 0.7 0.7Zn-PCTP-72 Dymalink ® 526 40 40 40 40 Perkadox ® BD-FF 2.0 2.5 1.5 2.02-nitrophenol 0.5 0.5 0.3 0.3 Properties Core Diameter (in) 1.55 1.5491.547 1.546 Core Weight (oz) 1.302 1.304 1.302 1.302 Compression 62 5678 74 (DCM) COR 0.794 0.791 0.804 0.796 Hardness at 50 52.3 55.8 54.2Geometric Center (Shore C) Hardness at 6 mm 67.2 70.2 75.5 74.8 fromCenter (Shore C) Hardness at 14 mm 66 65.2 71.4 71.6 from Center (ShoreC) Hardness at Core 89.4 91.5 91.3 92.2 Surface (Shore C) HardnessGradient 39.4 39.2 35.5 38 (Shore C)

As shown in Table 1, the hardness gradient generally increases as theconcentration of the hardening agent and/or initiator increases. Thehardness gradient appears to plateau as the hardness gradient approaches40 Shore C. Further, the hardness gradient from the geometric center ofthe core to 6 mm from the center of the core and from 14 mm from thecenter of the core to the surface of the core is typically greater thanthe hardness gradient from 6 mm from the center of the core to 14 mmfrom the center of the core.

Example 2

The following examples describe golf ball cores having a single layer(solid sphere) made from rubber formulations in accordance with thepresent invention. The rubber formulations below include a base rubbercomprised of a bend of polybutadiene rubbers, Buna® CB 1221 and Buna® CB24; a co-agent, Dymalink® 526 (zinc diacrylate); a first filler, ZnO; asecond filler, PolyWate® 325 (barium sulfate); a radical scavenger,Rhenogran® Zn-PCTP-72; an initiator, Perkadox® BD-FF (dicumyl peroxide);and a hardening agent, 3-nitrophenol. The compositions and properties ofthe cores are described in Table 2 below.

TABLE 2 Golf Ball Cores Including 3-Nitrophenol A B C D CompositionBuna ® CB 1221 85 85 85 85 Buna ® CB 24 15 15 15 15 Zinc Oxide 5 5 5 5Poly Wate ® 325 10.8 10.8 14.9 15 Rhenogran ® 0.7 0.7 0.7 0.7 Zn-PCTP-72Dymalink ® 526 40 40 30 30 Perkadox ® BD-FF 2 2.5 2 2.5 3-nitrophenol0.5 0.5 0.5 0.5 Properties Core Diameter (in) 1.55 1.547 1.558 1.553Core Weight (oz) 1.292 1.295 1.316 1.309 Compression 97 97 57 62 (DCM)COR 0.816 0.815 0.798 0.801 Hardness at 71.8 69.5 62.7 62.9 GeometricCenter (Shore C) Hardness at 6 mm 81.3 81.9 70.2 70.6 from Center (ShoreC) Hardness at 14 mm 80.8 75.8 72.7 72.1 from Center (Shore C) Hardnessat Core 84.1 90 78 83 Surface (Shore C) Hardness Gradient 12.3 20.5 15.320.1 (Shore C)

As shown in Table 2, the hardness gradient and compression generallyincrease as the concentration of the initiator increases suggesting asynergistic effect between the hardening agent and the initiator (at aminimum). The compression is also generally lower when lowerconcentrations of co-agent are used. Further, hardness gradient from thegeometric center of the core to 6 mm from the center of the core andfrom 14 mm from the center of the core to the surface of the core tendsto be greater than the hardness gradient from 6 mm from the center ofthe core to 14 mm from the center of the core.

Example 3

The following examples describe golf ball cores having a single layermade from rubber formulations in accordance with the present invention.The rubber formulations below include a base rubber comprised of a bendof polybutadiene rubbers, Buna CB 1221 and Buna® CB 24; a co-agent,Dymalink® 526 (zinc diacrylate); a first filler, ZnO; a second filler,PolyWate® 325 (barium sulfate); a radical scavenger, Rhenogran®Zn-PCTP-72; an initiator, Perkadox® BD-FF (dicumyl peroxide); and ahardening agent, 4-nitrophenol. The compositions and properties of thecores are described in Table 3 below.

Golf Ball Cores Including 4-Nitrophenol A B D E Composition Buna ® CB1221 85 85 85 85 Buna ® CB 24 15 15 15 15 Zinc Oxide 5 5 5 5 Poly Wate ®325 10.8 10.8 14.9 15 Rhenogran ® 0.7 0.7 0.7 0.7 Zn-PCTP-72 Dymalink ®526 40 40 30 30 Perkadox ® BD-FF 2.0 2.5 2.0 2.5 4-nitrophenol 0.5 0.50.5 0.5 Properties Core Diameter (in) 1.545 1.544 1.553 1.551 CoreWeight (oz) 1.301 1.304 1.315 1.313 Compression 117 113 77 78 (DCM) CORN/A N/A 0.803 0.801 Hardness at 80.6 79.8 70.8 71.6 Geometric Center(Shore C) Hardness at 6 mm 88.5 87.4 78.3 78.3 from Center (Shore C)Hardness at 14 mm 83.5 78.8 76.2 75.6 from Center (Shore C) Hardness atCore 89.9 92 81.3 83.2 Surface (Shore C) Hardness Gradient 9.3 12.2 10.511.6 (Shore C)

As shown in Table 3, the hardness gradient generally increase as theconcentration of the initiator increases suggesting a synergistic effectbetween the hardening agent and the initiator (at a minimum) and thecompression generally decreases as the concentration of co-agentincreases. Further, hardness gradient from the geometric center of thecore to 6 mm from the center of the core and from 14 mm from the centerof the core to the surface of the core is greater than the hardnessgradient from 6 mm from the center of the core to 14 mm from the centerof the core.

Example 4

The following examples describe golf ball cores having a single layermade from rubber formulations in accordance with the present invention.The rubber formulations include a base rubber comprised of a bend ofpolybutadiene rubbers, Buna® CB 1221 and Buna® CB 24; a co-agent,Dymalink® 526 (zinc diacrylate); a first filler, ZnO; a second filler,PolyWate® 325 (barium sulfate); a radical scavenger, Rhenogran®Zn-PCTP-72; an initiator, Perkadox® BD-FF (dicumyl peroxide). In theexamples below, different hardening agents are used. The hardeningagents include 2-nitrophenol (ortho), 3-nitrophenol (meta), and4-nitrophenol (para). In addition to data about the hardness, hardnessgradient, compression, COR, core weight, and core diameter, the examplesbelow also include spin data. For collecting spin data, the coresdescribed below were made into golf balls including an inner cover layerformed from ionomer and a thin polyurethane outer cover. Thecompositions, properties and spin data of example cores are shown belowTable 4.

TABLE 4 Golf Ball Cores Including Nitrophenol A B C Composition Buna ®CB 1221 85 85 85 Buna ® CB 24 15 15 15 Zinc Oxide 5 5 5 Poly Wate ® 32510.7 13.5 15.2 Rhenogran ® 0.7 0.7 0.7 Zn-PCTP-72 Dymalink ® 526 40 3329 Perkadox ® BD-FF 1.5 2 2 Nitrophenol Isomer Ortho Meta ParaNitrophenol 0.5 0.5 0.5 Core Core Diameter (in) 1.529 1.533 1.533Properties Core Weight (oz) 1.252 1.258 1.262 Compression (DCM) 71 72 73COR 0.801 0.803 0.802 Hardness at Geometric 50.3 64.7 70.3 Center(ShoreC) Hardness at 6 mm from 72.1 73.6 77.2 Center (Shore C) Hardness at 14mm from 69.8 75.6 76 Center (Shore C) Hardness at Core 88.3 79.6 78.8Surface (Shore C) Hardness Gradient 38 14.9 8.5 (Shore C) Ball BallDiameter (in) 1.683 1.683 1.683 Properties Ball Weight (oz) 1.602 1.5981.602 Compression (DCM) 99 95 93 COR 0.818 0.820 0.820 Spin Spin Ratewith Driver 2683 2681 2723 Data (rpm) Spin Rate with Hybrid 4155 41004266 (rpm) Spin Rate with Iron 6389 6386 6651 (rpm) Spin Rate with Full9213 9165 9418 Wedge (rpm) Spin Rate with Half 6905 6862 6910 Wedge(rpm)

As shown in Table 4, the hardness gradient of a core is generally higherif the hardening agent is 2-nitrophenol than if the hardening agent is3-nitrophenol and is generally higher if the hardening agent is3-nitrophenol than if the hardening agent is 4-nitrophenol. Table 4 alsoindicates that altering the distance between the substituent ondisubstituted benzoic hardening agent (i.e., changing the hardeningagent from 2-nitrophenol to 3-nitrophenol or 4-nitrophenol) generallyhas a greater effect on the hardness gradient than altering theconcentration of the initiator, however, this may not hold true forlarge changes (greater than those tested) in the initiatorconcentration. It is also noted that the compression of the golf ballcores shown in Table 4 are relatively similar despite changes in thehardening agent. Without being bound to any theory, this may beattributable to changes in the concentration of co-agent used withdifferent hardening agents to compensate for the hardening agent'seffect on core compression.

Unless otherwise defined, all terms (including technical and scientificterms) used herein have the same meaning as commonly understood by oneof ordinary skill in the art of this disclosure. It will be furtherunderstood that terms, such as those defined in commonly useddictionaries, should be interpreted as having a meaning that isconsistent with their meaning in the context of the specification andshould not be interpreted in an idealized or overly formal sense unlessexpressly so defined herein. Well known functions or constructions maynot be described in detail for brevity or clarity.

The terms “about” and “approximately” shall generally mean an acceptabledegree of error or variation for the quantity measured given the natureor precision of the measurements. Numerical quantities given in thisdescription are approximate unless stated otherwise, meaning that theterm “about” or “approximately” can be inferred when not expresslystated.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting. As used herein, thesingular forms “a”, “an” and “the” are intended to include the pluralforms as well (i.e., at least one of whatever the article modifies),unless the context clearly indicates otherwise.

The terms “first,” “second,” and the like are used to describe variousfeatures or elements, but these features or elements should not belimited by these terms. These terms are only used to distinguish onefeature or element from another feature or element. Thus, a firstfeature or element discussed below could be termed a second feature orelement, and similarly, a second feature or element discussed belowcould be termed a first feature or element without departing from theteachings of the disclosure. Likewise, terms like “top” and “bottom”;“front” and “back”; and “left” and “right” are used to distinguishcertain features or elements from each other, but it is expresslycontemplated that a top could be a bottom, and vice versa.

The golf balls described and claimed herein are not to be limited inscope by the specific embodiments herein disclosed, since theseembodiments are intended as illustrations of several aspects of thedisclosure. Any equivalent embodiments are intended to be within thescope of this disclosure. Indeed, various modifications of the device inaddition to those shown and described herein will become apparent tothose skilled in the art from the foregoing description. Suchmodifications are also intended to fall within the scope of the appendedclaims. All patents and patent applications cited in the foregoing textare expressly incorporated herein by reference in their entirety. Anysection headings herein are provided only for consistency with thesuggestions of 37 C.F.R. § 1.77 or otherwise to provide organizationalqueues. These headings shall not limit or characterize the invention(s)set forth herein.

What is claimed is:
 1. A golf ball, comprising: a core comprising: arubber formulation including a base rubber and a hardening agent havinga hardening agent concentration HA_(C) and a hardening agent isomernumber HA_(LN); a geometric center hardness H_(C); a surface hardnessH_(S); and a hardness gradient H_(Gr) equal to the difference betweenH_(C) and H_(S), wherein the hardening agent is nitrophenol and${\frac{{HA}_{C}*{HA}_{LN}}{1 - \frac{1}{H_{Gr}}} \geq 0.4};$ and acover layer disposed about the core.
 2. The golf ball of claim 1,wherein${0.6} < \frac{{HA}_{C}*{HA}_{LN}}{1 - \frac{1}{H_{Gr}}} < {{0.6}{3.}}$3. The golf ball of claim 1, wherein${1.5} < \frac{{HA}_{C}*{HA}_{LN}}{1 - \frac{1}{H_{Gr}}} < {1.8.}$ 4.The golf ball of claim 1, wherein${2.0} < \frac{{HA}_{C}*{HA}_{LN}}{1 - \frac{1}{H_{Gr}}} < {2.5.}$ 5.The golf ball of claim 1, wherein HA_(C) is about 0.1 to about 1.0 partsper hundred rubber.
 6. The golf ball of claim 1, wherein H_(C) is in therange of about 50 Shore C to about 85 Shore C, H_(S) is in the range ofabout 65 Shore C to about 95 Shore C, and H_(S) is greater than H_(C).7. The golf ball of claim 1, wherein the rubber formulation furthercomprises a co-agent, a filler, and an initiator.
 8. The golf ball ofclaim 1, wherein the rubber formulation further comprises a radicalscavenger.
 9. The golf ball of claim 1, wherein the base rubber ispolybutadiene rubber, butyl rubber, or a blend thereof.
 10. A golf ball,comprising: a core comprising: a rubber formulation including a baserubber, an initiator having an initiator concentration I_(C), and ahardening agent having a hardening agent concentration HA_(C) and ahardening agent isomer number HA_(LN); a geometric center hardnessH_(C); a surface hardness H_(S); and a hardness gradient H_(Gr) equal tothe difference between H_(C) and H_(S), wherein the hardening agent isnitrophenol and${\frac{{HA}_{C}*{HA}_{LN}}{1 - \frac{I_{c}}{H_{Gr}}} \geq 0.6};$ and acover layer disposed about the core.
 11. The golf ball of claim 10,wherein${{0.6}2} < \frac{{HA}_{C}*{HA}_{LN}}{1 - \frac{I_{C}}{H_{Gr}}} < {{0.6}{4.}}$12. The golf ball of claim 10, wherein${1.6} < \frac{{HA}_{C}*{HA}_{LN}}{1 - \frac{I_{C}}{H_{Gr}}} < {2.0.}$13. The golf ball of claim 10, wherein${2.4} < \frac{{HA}_{C}*{HA}_{LN}}{1 - \frac{I_{C}}{H_{Gr}}} < {2.7.}$14. The golf ball of claim 10, wherein HA_(C) is about 0.1 to about 1.0parts per hundred rubber.
 15. The golf ball of claim 10, wherein I_(C)is about 0.5 to about 3.0 parts per hundred rubber.
 16. The golf ball ofclaim 10, wherein H_(C) is in the range of about 50 Shore C to about 85Shore C, Hs is in the range of about 65 Shore C to about 95 Shore C, andHs is greater than H_(C).
 17. A golf ball, comprising: a corecomprising: a rubber formulation including a base rubber and a hardeningagent, wherein the hardening agent is a benzoic compound comprising afirst functional group that is a nitro functional group and a secondfunctional group that is selected from the group consisting of hydroxyl,amino, and sulfhydryl functional groups; a geometric center having ahardness; a geometric surface having a hardness; a hardness gradientequal to the difference in the geometric center hardness and the surfacehardness, wherein the hardness gradient is between 2 Shore C and 42Shore C; and a cover layer disposed about the core.
 18. The golf ball ofclaim 17, wherein the hardening agent is nitrophenol.
 19. The golf ballof claim 18, wherein the hardening agent is 2-nitrophenol and thehardness gradient is between 30 Shore C and 42 Shore C.
 20. The golfball of claim 18, wherein the cover layer comprises a material selectedfrom the group consisting of polyurethanes, polyureas, and hybrids,copolymers, and blends thereof.